The present invention relates to the detection of acoustic signals and, more particularly, to the use of scattering to modulate electromagnetic energy.
Conventional devices for sensing acoustic waves depended upon direct conversion of the waves into electrical energy via piezoelectric, electrodynamic, or magnetostrictive transducer elements (e.g., U.S. Pat. No. 3,903,497 by M. Stimler and Z. I. Slawsky). Usually signal processing was employed to enhance the signal to noise ratio. With the advent of coherent, monochromatic radiation (e.g., the laser), detection of the effect of acoustic waves upon locally supplied electromagnetic energy has been proposed. Method And Apparatus for Optically Detecting Acoustic Disturbances, U.S. Pat. No. 3,474,253 by B. V. Kessler, an example of one proposal, uses a laser to project a beam of light through the open sea onto a photomultiplier. Coherent, low frequency acoustic disturbances cause Brillouin scattering of the light from the acoustic waves. This has recently been extended to include scattering (either Rayleigh or Mie) from particles present in the sea water which are driven to oscillate at the frequency of the acoustic signal. The open sea however, is a dynamic acoustic conductor. Intrinsic in the signals detected by this and by other examples of the prior art are noise contributions due to such dynamic sources as scattering center (particle) turbulence and the flow of the conducting medium.
As used herein the term "light" is not limited to the visible spectrum but connotes electromagnetic radiation generally, without limitation by bandwidth.